INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

DIVISION TRIPLOBLASTICA, OR BILATERIA

SUBDIVISION SPIRALIA

PHYLUM POGONOPHORA

CLASS FRENULATA

This Class includes the majority of described pogonophoran species, found primarily at depths exceeding 3,000 m, at temperatures ranging from -3 to +13 °C and normal Water salinity. However, in northern seas, Frenulata have also been found in shallow waters. Some species, such as Siboglinum caulleryi, occur at depths ranging from 20 to 8,100 m.

The size of Frenulata ranges from 5 to 36 cm in length and 0.1-2.5 mm in width.

Anatomy and Morphology. The anterior, shortest body region bears the tentacular apparatus, which consists of one to two thousand relatively long tentacles (Fig. 190). The tentacles are arranged in a circle or horseshoe shape. In many species, they lie close to one another, whereas in Representatives of the genera Lamellisabella and Spirobranchia, they unite into tentacular plates, forming a tentacular cylinder in the former and a spirally coiled tube in the latter. The second, somewhat longer region bears two cuticular ridges called the frenulum (hence the class name from the Latin *frenum* - bridle). These ridges rest against the edges of the tube and serve as a support for the anterior body region. The longest is the third, trunk region. It comprises two parts: the anterior preannular part, which extends to special girdles consisting of two to three rows of denticulate setae, and the posterior postannular part, located behind the girdles. A broad ciliated band runs along the ventral side of the preannular part. The trunk is covered with numerous rounded or pyriform adhesive papillae. Often, these papillae bear small chitinoid plates. The papillae and denticulate setae serve to anchor the body against the inner wall of the tube. The fourth region, which terminates the body, is short like the first and exhibits external segmentation; it typically bears metamerically arranged setae involved in burrowing movements.

The tubes inhabited by Frenulata vary in Structure, but the anterior end of the tube is always broader, and the first body region with its tentacles protrudes outward through its opening. Only the fourth body region protrudes through the narrow posterior opening of the tube; it undergoes peristaltic contractions, functioning as a digging organ. This region breaks off easily along with the posterior part of the tube and thus remained unknown to researchers for a long time, as it remained embedded in the substrate when the animal was pulled out. Various sessile invertebrates—such as Sponges, hydrozoan polyps, and bryozoans—colonize the tubes of many species. Throughout its life, the posterior end of the tube is continuously built upon, allowing the animal to burrow deeper and deeper into the sediment.

Skin-Muscle Sac. The body, including the tentacles, is covered by a cuticle under which lies a single-layered epithelium containing numerous unicellular glands. Beneath the epithelium lie layers of circular Muscles and, more deeply, longitudinal muscles; the contraction of either muscle layer results in strong elongation or shortening of the body.

Body Cavity. Each body region has its own coelomic compartment (Fig. 192): in the first region, the coelom is unpaired and pouch- or horseshoe-shaped, with branches extending into all tentacles; the second through fourth regions contain paired coelomic sacs. Two pairs of coelomoducts function as Kidneys and genital ducts.

Fig. 192. Diagram of the Internal Structure of the anterior body region of frenulate pogonophorans (from Ivanov): a - dorsal view; b - left view:

1, 2 - coelomic and Blood channels of the tentacles, respectively; 3 - coelom of the first body region; 4 - coelomoduct; 5 - second body region; 6 - its coelom; 7 - coelom of the third body region; 8, 9 - ventral and dorsal Blood Vessels, respectively; 10 - main blood vessel; 11 - Brain; 12 - lateral blood vessel; 13 - nerve trunk; 14 - vas deferens; 15 - origin of tentacular nerves;

16 - Pericardium; 17 - Heart; 18 - male genital opening

Adult animals lack a digestive tract. The question of how pogonophorans feed was debated for a long time. It has now been established that, like certain other marine invertebrates (Mollusks, polychaetes, Echinoderms, etc.), pogonophorans absorb organic matter dissolved in water, particularly Amino Acids, which penetrate their bodies through the body surface, including the tentacles.

In smaller forms, the Uptake of Amino acids accounts for a significant portion of their overall METABOLISM. Additionally, another unique feeding mechanism was discovered—initially in representatives of the class Aphrenoidea (Afrenulata) and later in certain Frenulata—via Chemosynthesis, owing to their Symbiosis with sulfur- and methane-oxidizing Bacteria. For instance, studies of small pogonophorans, particularly of the genera Siboglinum and Oligobranchia living on the ordinary seabed, have shown that their postannular trunk region contains a median channel (a remnant of the midgut) filled with symbiotic methane-oxidizing bacteria. These bacteria oxidize methane and utilize the generated energy to synthesize the Organic compounds required by pogonophorans. The habitats of Frenulata are rich in methane seeping from submarine oil and gas deposits.

The excretory system is represented by a pair of long ciliated canals (coelomoducts) connecting the coelom of the first body region to the external environment.

The Circulatory system is closed. Blood flows through the dorsal vessel toward the anterior end of the body; near the Base of the tentacles, the vessel dilates into a muscular heart, whose contractions force blood into the afferent tentacular vessels. These vessels break down into capillaries, where gas exchange primarily takes place. Subsequently, blood passes via efferent vessels into the ventral vessel and flows backward. In the posterior part of the trunk region, the dorsal and ventral vessels are connected by numerous transverse vessels. Pogonophoran blood is red and contains Hemoglobin.

Respiratory Organs are absent; gas exchange occurs mainly through the tentacles, which bear rows of numerous, extremely delicate, tiny outgrowths called pinnules (Figs. 191, 193). A pinnule is an expanded epithelial Cell containing two blood capillaries: an afferent capillary and an efferent capillary, which carries oxygenated blood to the efferent tentacular vessel.

The Nervous system lies within the epidermal epithelium and is represented by a nerve plexus containing condensations of Nerve Cells and fibers: a brain located on the ventral side of the first body region that innervates the tentacles, and a longitudinal nerve cord extending along the entire body.

Fig. 193. Cross-section through the tentacular plates of Spirobranchia grandis (some tentacles on the ventral side have been excised) (from Gruner)

Sense Organs are absent, their function being performed by tentacles. Reproductive System. Gonads in both males and females are located in the trunk region of the body. In females, a pair of Ovaries is situated in the anterior part of the trunk; mature oocytes are released through ruptured ovarian walls into the coelom and expelled from the body via oviducts (coelomoducts). The female deposits eggs into the anterior section of her own tube, where Fertilization and embryonic development take place. In males, the posterior half of the same region contains two long Seminal Vesicles from which two sperm ducts extend toward the anterior end of the trunk, opening on the DORSAL SIDE OF the body. Numerous flattened or spindle-shaped spermatophores with a long filament form within the sperm ducts and actively migrate from males to females.

Reproduction. The female deposits eggs into the anterior section of her own tube, where fertilization and embryonic development take place. Cleavage is total, unequal, and spiral. During the gastrula stage, a temporary gut rudiment appears, from which a pair of coelomic sacs pinches off (enterocoely); a blastopore is also present. Subsequently, the blastopore disappears, the endodermal rudiment disintegrates, while the coelomic sacs persist. The embryo gradually elongates and divides into four regions, corresponding to the division of the coelom.

The larva hatching from the egg consists of four segments and bears two ciliary bands (Fig. 194). The cilia subsequently disappear, and the larva grows in length, particularly its third region.

Fig. 194. Larva of Siboglinum florditnim (after Ivanov):

1 - anterior ciliary band; 2 - ventral ciliary strip;

3 - posterior ciliary band; 4 - setae; 5 - annular groove between the second